I. Field of the Invention:
This invention relates generally to electronic test apparatus, and more specifically to a device for determining the amplitude threshold at which an electronic device will respond to an input stimulus.
II. Discussion of the Prior Art:
The present invention will be described in association with cardiac pacer apparatus in that it has been particularly developed for use in testing and evaluating heart pacers. It is to be understood at the outset, however, that the present invention may be used in conjunction with other electronic apparatus where threshold sensitivity is the parameter to be measured or monitored.
As is well known in the art, a demand-type cardiac pacer can commonly be used in the treatment of cardiac irregularities on a noncompeting basis. As such, demand-type cardiac pacers include a pulse generator for producing output stimulating pulses at a predetermined rate in the absence of normal cardiac (R-wave) activity. The stimulating impulses are generally applied to the heart muscle by way of heart contacting electrodes. The same electrodes used to apply the stimulating impulses are also arranged to pick up the R-wave impulses generated during the depolarization of the heart muscle and the resulting signal is processed and employed to reset the pulse generator's timer mechanism such that the next pacer pulse will not appear on schedule, but instead, only after a predetermined time has elapsed since the detection of a preceding R-wave.
Not every impulse picked up by the heart contacting electrodes is a R-wave. Accordingly, cardiac pacer devices normally include circuitry for discriminating between R-waves and other electrical activity picked up by the electrodes. This discriminating circuitry may include band-pass filter circuits having a center frequency tuned to the normal frequency of a R-wave and, in addition, it may include a thresholding device requiring the detected signal to exceed a predetermined amplitude before it will be recognized as a true R-wave. Because of the inclusion of such threshold circuits, the attending physician has a need to measure the sensitivity or level at which a received R-wave will be capable of resetting the pacer pulse generator to thereby inhibit the generation of a pacer pulse. The present invention provides an apparatus for testing or measuring the sensitivity parameter of a pacemaker and for providing a visual display of the voltage amplitude of a waveform which is found to be just sufficient to pass the threshold criteria.
Various threshold analyzing devices are known in the art. For example, reference is made to the Herrmann U.S. Pat. No. 3,757,790, the Thaler Pat. No. 3,837,348, the Gombrich et al U.S. Pat. No. 3,920,005. While each of the foregoing patents relates in some way to apparatus for measuring the performance characteristics of cardiac pacer devices, none describes a system whereby the sensitivity characteristic of a pacer under test can be measured and displayed.